Add property updates to loaders

[doxel-sander]

It's currently impossible to declare properties on the results of resources loaded with loaders. For example:

<three-mesh>
    <plane-geometry />
    <mesh-basic-material>
        <!-- I want to set the texture's anisotropy to 32, for example, but can't do so declaratively -->
        <texture-loader src="/example.png" attach="map" />
    </mesh-basic-material>
</three-mesh>

Update so that this behaves as expected:

<three-mesh>
    <plane-geometry />
    <mesh-basic-material>
        <!-- Any non-`src` and non-`attach` attribute should be set on the loaded resource -->
        <!-- The loaded resource itself should also be the element's `.instance` rather than the loader - we can set the loader to a new property `.loader` -->
        <texture-loader src="/example.png" attach="map" anisotropy="32" />
    </mesh-basic-material>
</three-mesh>

[doxel-sander]

Done in #73